WASHINGTON -- Pledging to "put everything I've got into this," a somber President Obama unveiled the most sweeping proposals for curbing gun violence in two decades, pressing a reluctant Congress to pass universal background checks and bans on military-style assault weapons and high-capacity ammunition magazines like the ones used in the Newtown, Conn., school shootings.

Surrounded by children who wrote him letters seeking curbs on guns, Obama committed himself to a high-profile and politically volatile campaign behind proposals assembled by Vice President Joe Biden that will test the administration's strength heading into the next four years. The first big push of Obama's second term, then, will come on an issue that was not even on his to-do list on Election Day when voters renewed his lease on the presidency.

The emotionally charged ceremony, attended by family members of those killed at Sandy Hook Elementary School, reflected a decision by the White House to seize on public outrage to challenge the political power of the National Rifle Association and other forces that have successfully fought new gun laws for decades.

The plan, which includes 23 executive actions, was described as a major initiative by advocates on both sides of the debate. But in many respects, it is limited in scope, reflecting the political constraints of an issue that deeply divides the country, as well as the power of the gun lobby.

Left out, for example, was a proposal for background checks on buyers of ammunition, which Sen. Richard Blumenthal, D-Conn., a proponent, called "the black hole of gun violence prevention." Such checks were included in a sweeping New York law that Democratic Gov. Andrew Cuomo signed Tuesday.

Charles Dharapak, Associated Press

From left, Hinna Zeejah, 8, Taejah Goode, 10, Julia Stokes, 11, and Grant Fritz, 8, who wrote letters to President Obama about the school shooting in Newtown, Conn., watched Wednesday as Obama signed executive orders outlining his gun control proposals.

The most important parts of Obama's plan will require congressional approval. They include a federal ban on the sale of military-style assault weapons, with fewer loopholes than the 1994 law that expired in 2004. Several states already have such bans. The president also wants to reinstate an earlier ban on sales of magazines that can hold more than 10 rounds.

Obama also wants to expand the background-check system to encompass all gun purchases, including the nearly 40 percent that are estimated to occur at gun shows and in private sales. Six states require background checks on all firearms sold at gun shows, according to the Coalition to Stop Gun Violence.

Federally licensed gun dealers are already required to run checks to ensure that potential buyers have not been convicted of a felony or domestic violence, or committed to a mental institution. But for years, the National Rifle Association and its allies in Congress have blocked universal background checks.

"This will be difficult," Obama said. "There will be pundits and politicians and special-interest lobbyists publicly warning of a tyrannical, all-out assault on liberty -- not because that's true, but because they want to gin up fear, or higher ratings, or revenue for themselves. And behind the scenes, they'll do everything they can to block any common-sense reform."

Obama said his response to that opposition will be to try to mobilize public support. "I tell you, the only way we can change is if the American people demand it," Obama said. "And by the way, that doesn't just mean from certain parts of the country. We're going to need voices in those areas, in those congressional districts where the tradition of gun ownership is strong, to speak up and to say this is important. It can't just be the usual suspects."

He urged the public to pressure lawmakers. "Ask them what's more important, doing whatever it takes to get an A grade from the gun lobby that funds their campaigns, or giving parents some little peace of mind when they drop their children off for first grade," he said.

In a tacit acknowledgment of the political limits, Obama signed three executive orders that do not require congressional approval to enhance the tracing of weapons seized by law enforcement, provide more federal records to the background-check system and foster research into gun violence.

He condemned previous efforts by Congress, prodded by the gun lobby, that have effectively blocked the Centers for Disease Control and Prevention from investigating the causes of gun violence.

"We don't benefit from ignorance. We don't benefit from not knowing the science of this epidemic of violence," said Obama, who also called on Congress to "fund research into the effects that violent video games have on young minds."

Obama announced numerous initiatives. He said the background-check system would be strengthened by, among other steps, making it easier for states to share information about mentally ill persons who should be prohibited from owning guns. He also proposed spending for increased training in the areas of school safety and mental health.

The price tag of the package is nearly $4.5 billion, the White House said. Most of it -- $4 billion -- would subsidize the cost of keeping 15,000 police on the streets, renewing a portion of an earlier Obama jobs initiative that failed to gain approval in Congress.

Foes of gun control condemned Obama's actions, calling them an infringement of the rights of gun owners and an ineffective response to gun violence. Typical was the response of Sen. Marco Rubio, a potential 2016 Republican presidential candidate, who said Obama "is again abusing his power by imposing his policies via executive fiat instead of allowing them to be debated in Congress."

The NRA echoed its criticisms of Obama. "Attacking firearms and ignoring children is not a solution to the crisis we face as a nation," the group said in a statement. "Only honest, law-abiding gun owners will be affected and our children will remain vulnerable to the inevitability of more tragedy."

For Obama, the plan represented a political pivot. While he has always expressed support for an assault weapons ban, he has made no real effort to pass it on the assumption that the votes were not there. But he and the White House are banking on the idea that the Newtown shooting has changed the dynamics.

"I have never seen the nation's conscience so shaken by what happened at Sandy Hook," Biden said. "The world has changed and is demanding action."

The Associated Press and New York Times contributed to this report.

WHAT'S NEXT

Next week: Legislation expected to be introduced to renew the assault weapons ban and limit ammunition magazines to 10 rounds.

Coming weeks: President Obama and Vice President Joe Biden will take the campaign on the road in the coming weeks as they seek to build public pressure on Congress to act on their proposals. NRA signals it's ready to fight the proposal, but doesn't give specifics.

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